The copper vapor laser is an efficient source of visible radiation of high average power. Because the laser operates on a transition, the lower level of which is metastable, efficient operation requires a rapid pumping current pulse, and repetitive operation depends on depopulation of the metastable level primarily by collisional means. The majority of high power and efficient copper vapor lasers have utilized discharges in ceramic cylindrical tubes that are up to several meters in length.
The gas discharge, with an inert gas such as neon or helium as the buffer gas, takes place longitudinally along the length of the lasing volume in the ceramic tube between two electrode assemblies located at each end of the ceramic tube. Typically, a metal housing which serves as a coaxial return for the discharge current surrounds the ceramic tube. Near-normal incidence quartz windows held in water cooled mounts terminate the two ends of the vacuum enclosure and serve as the optical aperture windows. Copper vapor is introduced into the discharge at reservoirs located around the inside of the ceramic tube and the extent of the lasing region is determined by the axial temperature profile of the ceramic tube. The laser operates in the self-heated mode in which the discharge power that excites the laser medium also acts as the power source to heat the tube to its operating temperature.
Attempts have been made to increase the power output of such a laser by increasing the diameter of the ceramic tube to thereby increase the lasing volume within the tube. However, as the diameter is increased, the gas temperature increases. This trend continues until a diameter is reached where no further increase in output power is possible because the gas temperature has increased to a level wherein the metastable energy level is thermally populated to such a level that the population inversion necessary for lasing is significantly reduced or not even achieved in much of the lasing volume.
In an attempt to overcome the heating problem in large diameter lasing tubes, an inner ceramic tube has been placed inside the ceramic lasing tube, coaxially therewith, so that an elongated annular lasing volume is formed between the two tubes. Such a structure allows cooling of the gas by wall collisions and gas conduction to the inner ceramic tube. This tube is cooled by thermal radiation to the externally cooled outer ceramic tube. This allows a higher gas discharge volume with increased power density while lowering the gas temperature, and therefore provides more laser output power.
The inner and outer tubes are typically made of ceramic material for two reasons. First, ceramic materials have very high electrical resistivity and thus will confine the current-flow between the electrodes to the annular volume between the two tubes without electrically shorting out such flow. Secondly, suitably chosen ceramic materials will normally resist the high operating temperature, in the order of 1500xc2x0 C., in a copper vapor laser.
Although the coaxial tube arrangement does provide cooling of the gas between the tubes so that the system can be scaled up in size for increased power output, significant mechanical problems exist because of breakage of the inner and/or outer ceramic tubes. One reason for such failure is that the lasing is often not uniform around the annular lasing volume, but can be concentrated through a small portion of the annulus. Such concentration produces localized heating of the inner and outer tubes. The unevenness of heating causes cracking and eventual destruction of one or both tubes. Another cause of failure is thermal cycling on start up and shut down. This induces thermal shock failures, such as cracking of the ceramic tube.
It is the principal object of the invention to provide a metal vapor laser having an elongated ceramic tube with an internal structure that will provide an annular lasing volume and will have a much greater resistance to mechanical failure than the inner structures heretofore used.
Additional objects, advantages and novel features will be set forth in the description which follows, and in part will become apparent to those skilled in the art upon examination of the following, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of instrumentalities and combinations pointed out in the appended claims.
To achieve the foregoing and other objects, and in accordance with the present invention as described and broadly claimed herein, an inner structure is provided for a metal vapor laser having an elongated ceramic cylindrical tube whose inner surface defines the outer boundary of a lasing volume extending along the length of the ceramic tube, said laser also having electrode assemblies at each end of the ceramic tube, the inner structure comprising a plurality of circular metal members each having a diameter less than the diameter of the inner surface of the ceramic tube, and means for mounting the metal members coaxially in the ceramic tube, with the metal members being spaced physically and electrically from each other along the length of the ceramic tube.